Quick question, what size bolt is this?

Mark A

Mark A

Mark A

Soldato
Joined
19 May 2005
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Location
Lancashire
Need to order a new bolt for my airgun, but cant seem to find out what the current bolt is. If the OD inc the threads is 4.5mm, is that an M4 or M5 bolt? I thought it was supposed to be either 4 or 5mm, but not bang in the middle.

Thanks
 
M5 is a nominal 5mm across the threads. M4 is 4mm across threads. There is such a thing as an M4.5, but it's not as common.

What are you measuring with?

Apologies for the background colour (migraine inducing), but here's a thread chart:
http://mdmetric.com/tech/M-thead 600.htm
The major diameter is what you're measuring BTW. The minor diameter is the diameter minus the threads.
 
Argh, I cant actually look at that for more than a few seconds :p.

DSC_4042_zpswttayfud.jpg


Just measuring with some cheapo digital callipers. So I think this is possibly an M5 bolt then? Its a BSA Lightning that is about 10 years old, so i'm pretty sure it will me a metric bolt.
 
DSC_4044_zpsxwel3dvu.jpg


My eyes are getting bad, I had to take a photo and blow it up. i count 32 threads per inch. Which I think works out near 0.75mm, so it be the M4.5bolt you mentioned. The only problem is I cant even find any for sale on ebay.

I think i'm just going to have to buy one from somewhere like Chambers for like £5 inc del :(. Ideally i wanted to use an off the shelf bolt that isn't made out of cheese like the BSA bolt was.

The other option is drilling out the cylinder end plug and then re-tapping it to accept M6 or something. At least then I can easily replace it in future if it strips again. I'll just have to check the plug isn't hardened.
 
If that's an inch between the jaws in the pic above, you're looking at nearer 31 TPI and you've miscounted the threads (count the valleys, not the peaks). That gives a pitch of ~0.81mm. An M4.5 screw will never be over 4.5mm in diameter.

As Meridian suggested, that looks very much like an imperial thread - 2BA. The nominal size over the thread for 2BA is Ø4.7mm. 2BA pitch is 0.81mm.

Second table in the link:

http://www.ring-plug-thread-gages.com/PDChart/BA-thread-data.html

Major diameter (OD) of 2BA can be between 4.675mm and 4.151mm.

This link shows metric thread sizes:

http://mdmetric.com/tech/M-thead 600.htm

Where M4.5 is between 4.478 and 4.338mm.

Of course, if your caliper is not zeroed, then that could be an issue...
 
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Thanks for the help all. The calliper was £15 or so from ebay so the chances of it being accurate are probably not very good :o.

Well 2BA seem a lot easier to get hold of than M4.5, so I might give one a try. It would save a lot of messing about if it is right. I can get 30 for £4 for a lifetime supply if they fit :D

My gun is around 10 years old, but according to the serial number I think the design dates back to the 80s, so there's a good chance they are imperial after all.

EDIT: I think I have confirmed it. While googling 2BA it appears that darts still use this thread to connect the shaft to the head and lo and behold the original bolt is a perfect match in my darts :).
 
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Take it to an engineer's merchant and ask for one like it. They'll measure it up and tell you for sure. (Though, as you say, it does sound like 2BA).

Where abouts in Lancashire are you? I might be able to recommend a couple of places to try...
 
Thanks but i think its all sorted now. Ordered 2 x 2BA bolts last night from ebay and they were despatched at 7am this morning 1st class, so should be able to use the gun over the bank holiday.

Have to say this subforum is really helpful. I wouldn't have expected to get an answer at 11pm on a computer forum :cool:. I almost just ordered some M5 bolts, so I am glad I asked here first.
 
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